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OPEN
Received: 26 October 2016 Accepted: 13 March 2017 Published: xx xx xxxx
Hybrid Organic-Inorganic Perovskite Memory with Long-Term Stability in Air Bohee Hwang & Jang-Sik Lee Organic-inorganic perovskite materials have attracted extensive attention for wide range of applications such as solar cells, photo detectors, and memory devices. However, the lack of stability in ambient condition prevented the perovskite materials from applying to practical applications. Here, we demonstrate resistive switching memory devices based on organic-inorganic perovskite (CH3NH3PbI3) that have been passivated using thin metal-oxide-layers. CH3NH3PbI3-based memory devices with a solution-processed ZnO passivation layer retain low-voltage operation and, on/off current ratio for more than 30 days in air. Passivation with atomic-layer-deposited (ALD) AlOx is also demonstrated. The resistive switching memory devices with an ALD AlOx passivation layer maintained reliable resistive switching for 30 d in ambient condition, but devices without the passivation layer degraded rapidly and did not show memory properties after 3 d. These results suggest that encapsulation with thin metaloxide layers is easy and commercially-viable methods to fabricate practical memory devices, and has potential to realize memory devices with long-term stability and reliable, reproducible programmable memory characteristics. Organic-inorganic perovskite (OIP) materials are widely used in electronic and optoelectronic devices including light-emitting diodes1, 2, photo detectors3, 4, and lasers5, 6 and in solar cells7, 8. OIPs contain defects, which migrate when subjected to an electrical field; as a result this material exhibits sweep-dependent hysteresis in current-voltage (I − V) responses9. This ion migration in a perovskite layer can form a reversible p-i-n structure, in which photocurrent direction can be switched by applying a small electric field10. Moreover, organic cations that can rotate under an applied electrical field show ferroelectric behavior by positive and negative poling, and have structural flexibility11. These properties of OIP materials suggest applications as computer memory application12–16. However, OIPs are not stable in humidity and ambient atmosphere, so devices break down quickly17. For this reason, OIP films should be fabricated in N2 atmosphere, and devices that are not encapsulated cannot last long in the ambient atmosphere18, 19; this characteristic impedes commercialization and application of OIP electronic devices. To improve the long-term stability of OIP solar cells, various approaches have been tested. For example, ultrathin Al2O3 layers on the OIP layer isolate the perovskite layer from moisture, and thereby increase device stability20. Hydrophobic oligothiophene hole transport layers (HTLs) have been used as a protective layer for OIP film21. Solution-processed ZnO nanoparticle (NP) film that functions as an electron transport layer improved the efficiency and the stability of the cell22. However, the effect of passivating OIP films for memory applications has not been studied. In this study, we selected ZnO and AlOx as the protecting layer to protect OIPs from degradation by moisture and air. ZnO is already used as an air-stable cathode in polymer light emitting diodes23, and has been applied in solar cells as charge transport layer that also shields the photoactive layer from the ambient air24. Chemically-modified ZnO nanorods are water- resistant due to nanostructures with low surface energies that yield high contact angles with water droplets25. Al2O3 layers fabricated using atomic layer deposition (ALD) have been used as protective coatings for copper26, and as gas-diffusion barriers for polymer substrates27. These features may protect the perovskite device from moisture. We fabricated air-stable OIP (CH3NH3PbI3)-based ReRAM devices passivated by metal-oxide layers that are deposited by different methods such as solution process and ALD process. Devices without the passivation layer degraded after exposure to ambient air for less than 3 d. Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea. Correspondence and requests for materials should be addressed to J.-S.L. (email: jangsik@ postech.ac.kr)
Scientific Reports | 7: 673 | DOI:10.1038/s41598-017-00778-5
1
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Figure 1. Schematic of (a) Au/CH3NH3PbI3/ITO devices and (b) Au/ZnO/CH3NH3PbI3/ITO devices. Crosssectional images of (c) Au/CH3NH3PbI3/ITO devices and (d) Au/ZnO/CH3NH3PbI3/ITO devices. Plan views of perovskite (e) without ZnO film and (f) with ZnO film.
In contrast, all-solution-processed Au/ZnO/CH3NH3PbI3/ITO memory devices showed reliable operation for 30 d in ambient air, and Al/ALD_AlOx/CH3NH3PbI3/ITO memory devices showed bipolar resistive switching property for 30 d in ambient air. This concept of with metal-oxide layer passivation could realize perovskite memory devices that work stably in ambient air.
Results and Discussion
Au/CH3NH3PbI3/ITO-coated glass and Au/ZnO/CH3NH3PbI3/ITO-coated glass were used to demonstrate memory devices that have a metal/insulator/metal structure (Fig. 1a,b). A two-step spin coating method28 was used to coat CH3NH3PbI3 layer on a PbI2 surface. The CH3NH3PbI3 layer synthesized on ITO-coated glass formed a uniform film of thickness ~218 nm (Fig. 1c). The ZnO permeation barrier was then formed by repeating spin-coating of ZnO NPs (dispersed in chlorobenzene) on the CH3NH3PbI3 layer22, 29. Photomicrographs of the perovskite film with ZnO NPs (Au electrode not included) (Fig. 1d) show individual layer of ZnO (105 nm thick) and CH3NH3PbI3 (196 nm thick). The perovskite film without ZnO layer consisted of dense and closely-packed grains with the sizes of 100–200 nm. (Fig. 1e), and the perovskite film capped by ZnO layer was homogenously covered with ZnO NPs (Fig. 1f). The electrical properties of Au/CH3NH3PbI3/ITO and Au/ZnO/CH3NH3PbI3/ITO devices were characterized under ambient conditions; in both devices, the measured I − V curves exhibited bipolar resistive switching under compliance current of CC = 1 mA (Fig. 2a,b). To measure the I − V characteristics of the Au/CH3NH3PbI3/ITO device and the Au/ZnO/CH3NH3PbI3/ITO device, the voltage was controlled by one of the Au electrodes under dc sweeping voltage applied as 0 V → 2 V → 0 V → −1.5 V → 0 V; the bottom electrode (ITO) was grounded. In the CH3NH3PbI3-based device without ZnO layer, during the first voltage sweep from 0 V to set voltage Vset~1.1 V, Scientific Reports | 7: 673 | DOI:10.1038/s41598-017-00778-5
2
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Figure 2. Resistive switching characteristics of hybrid OIP based devices. (a) Au/CH3NH3PbI3/ITO/glass devices and (b) Au/ZnO/CH3NH3PbI3/ITO/glass devices.
positively-charged iodine vacancies migrate toward the negatively-charged electrode (ITO) to form conductive filaments that transport carriers injected from electrodes15. After the conductive filaments formed, the resistance state changed from high-resistance state (HRS) (OFF state) to low-resistance state (LRS) (conductive ON state). When a negative voltage was applied, the current decreased gradually at reset voltage Vreset
OPEN
Received: 26 October 2016 Accepted: 13 March 2017 Published: xx xx xxxx
Hybrid Organic-Inorganic Perovskite Memory with Long-Term Stability in Air Bohee Hwang & Jang-Sik Lee Organic-inorganic perovskite materials have attracted extensive attention for wide range of applications such as solar cells, photo detectors, and memory devices. However, the lack of stability in ambient condition prevented the perovskite materials from applying to practical applications. Here, we demonstrate resistive switching memory devices based on organic-inorganic perovskite (CH3NH3PbI3) that have been passivated using thin metal-oxide-layers. CH3NH3PbI3-based memory devices with a solution-processed ZnO passivation layer retain low-voltage operation and, on/off current ratio for more than 30 days in air. Passivation with atomic-layer-deposited (ALD) AlOx is also demonstrated. The resistive switching memory devices with an ALD AlOx passivation layer maintained reliable resistive switching for 30 d in ambient condition, but devices without the passivation layer degraded rapidly and did not show memory properties after 3 d. These results suggest that encapsulation with thin metaloxide layers is easy and commercially-viable methods to fabricate practical memory devices, and has potential to realize memory devices with long-term stability and reliable, reproducible programmable memory characteristics. Organic-inorganic perovskite (OIP) materials are widely used in electronic and optoelectronic devices including light-emitting diodes1, 2, photo detectors3, 4, and lasers5, 6 and in solar cells7, 8. OIPs contain defects, which migrate when subjected to an electrical field; as a result this material exhibits sweep-dependent hysteresis in current-voltage (I − V) responses9. This ion migration in a perovskite layer can form a reversible p-i-n structure, in which photocurrent direction can be switched by applying a small electric field10. Moreover, organic cations that can rotate under an applied electrical field show ferroelectric behavior by positive and negative poling, and have structural flexibility11. These properties of OIP materials suggest applications as computer memory application12–16. However, OIPs are not stable in humidity and ambient atmosphere, so devices break down quickly17. For this reason, OIP films should be fabricated in N2 atmosphere, and devices that are not encapsulated cannot last long in the ambient atmosphere18, 19; this characteristic impedes commercialization and application of OIP electronic devices. To improve the long-term stability of OIP solar cells, various approaches have been tested. For example, ultrathin Al2O3 layers on the OIP layer isolate the perovskite layer from moisture, and thereby increase device stability20. Hydrophobic oligothiophene hole transport layers (HTLs) have been used as a protective layer for OIP film21. Solution-processed ZnO nanoparticle (NP) film that functions as an electron transport layer improved the efficiency and the stability of the cell22. However, the effect of passivating OIP films for memory applications has not been studied. In this study, we selected ZnO and AlOx as the protecting layer to protect OIPs from degradation by moisture and air. ZnO is already used as an air-stable cathode in polymer light emitting diodes23, and has been applied in solar cells as charge transport layer that also shields the photoactive layer from the ambient air24. Chemically-modified ZnO nanorods are water- resistant due to nanostructures with low surface energies that yield high contact angles with water droplets25. Al2O3 layers fabricated using atomic layer deposition (ALD) have been used as protective coatings for copper26, and as gas-diffusion barriers for polymer substrates27. These features may protect the perovskite device from moisture. We fabricated air-stable OIP (CH3NH3PbI3)-based ReRAM devices passivated by metal-oxide layers that are deposited by different methods such as solution process and ALD process. Devices without the passivation layer degraded after exposure to ambient air for less than 3 d. Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea. Correspondence and requests for materials should be addressed to J.-S.L. (email: jangsik@ postech.ac.kr)
Scientific Reports | 7: 673 | DOI:10.1038/s41598-017-00778-5
1
www.nature.com/scientificreports/
Figure 1. Schematic of (a) Au/CH3NH3PbI3/ITO devices and (b) Au/ZnO/CH3NH3PbI3/ITO devices. Crosssectional images of (c) Au/CH3NH3PbI3/ITO devices and (d) Au/ZnO/CH3NH3PbI3/ITO devices. Plan views of perovskite (e) without ZnO film and (f) with ZnO film.
In contrast, all-solution-processed Au/ZnO/CH3NH3PbI3/ITO memory devices showed reliable operation for 30 d in ambient air, and Al/ALD_AlOx/CH3NH3PbI3/ITO memory devices showed bipolar resistive switching property for 30 d in ambient air. This concept of with metal-oxide layer passivation could realize perovskite memory devices that work stably in ambient air.
Results and Discussion
Au/CH3NH3PbI3/ITO-coated glass and Au/ZnO/CH3NH3PbI3/ITO-coated glass were used to demonstrate memory devices that have a metal/insulator/metal structure (Fig. 1a,b). A two-step spin coating method28 was used to coat CH3NH3PbI3 layer on a PbI2 surface. The CH3NH3PbI3 layer synthesized on ITO-coated glass formed a uniform film of thickness ~218 nm (Fig. 1c). The ZnO permeation barrier was then formed by repeating spin-coating of ZnO NPs (dispersed in chlorobenzene) on the CH3NH3PbI3 layer22, 29. Photomicrographs of the perovskite film with ZnO NPs (Au electrode not included) (Fig. 1d) show individual layer of ZnO (105 nm thick) and CH3NH3PbI3 (196 nm thick). The perovskite film without ZnO layer consisted of dense and closely-packed grains with the sizes of 100–200 nm. (Fig. 1e), and the perovskite film capped by ZnO layer was homogenously covered with ZnO NPs (Fig. 1f). The electrical properties of Au/CH3NH3PbI3/ITO and Au/ZnO/CH3NH3PbI3/ITO devices were characterized under ambient conditions; in both devices, the measured I − V curves exhibited bipolar resistive switching under compliance current of CC = 1 mA (Fig. 2a,b). To measure the I − V characteristics of the Au/CH3NH3PbI3/ITO device and the Au/ZnO/CH3NH3PbI3/ITO device, the voltage was controlled by one of the Au electrodes under dc sweeping voltage applied as 0 V → 2 V → 0 V → −1.5 V → 0 V; the bottom electrode (ITO) was grounded. In the CH3NH3PbI3-based device without ZnO layer, during the first voltage sweep from 0 V to set voltage Vset~1.1 V, Scientific Reports | 7: 673 | DOI:10.1038/s41598-017-00778-5
2
www.nature.com/scientificreports/
Figure 2. Resistive switching characteristics of hybrid OIP based devices. (a) Au/CH3NH3PbI3/ITO/glass devices and (b) Au/ZnO/CH3NH3PbI3/ITO/glass devices.
positively-charged iodine vacancies migrate toward the negatively-charged electrode (ITO) to form conductive filaments that transport carriers injected from electrodes15. After the conductive filaments formed, the resistance state changed from high-resistance state (HRS) (OFF state) to low-resistance state (LRS) (conductive ON state). When a negative voltage was applied, the current decreased gradually at reset voltage Vreset
